[clug] long run function in trolltech qt widget

Daniel Pittman daniel at rimspace.net
Fri Dec 5 03:18:35 GMT 2008

jm <jeffm at ghostgun.com> writes:
> Daniel Pittman wrote:
>>> You could use a QThread (its not hard, but make sure you read the
>>> docs).
>> Hah.  This, to me, seems like a rather risky statement: the original
>> poster stated that this was a wrapped C library, which starts us off on
>> an exciting foot.
>> Are you certain that the library doesn't make unsafe use of a signal
>> handler?  How about those non-entrant libc functions?  Do /you/ want to
>> explain appropriate locking to protect shared data structures?
> Luckily this assumption holds.

Now, see, this is one of the most problematic aspects of threading: are
you *sure* it really works, or is it just that there is a 1 in 1000
chance that it will crash?

Are you sure that it doesn't receive a corrupt response (or assertion
failure) from libc when it happens to just sneak in under the wire?

Worse, are you *sure* this will be true tomorrow, when you add a cool
new feature to your wizard, or when the library is enhanced?

> I got the thread version going yesterday, but the problem now is that
> it runs multiple times.  The thread start method it called from within
> the wizard's page showevent() method, but it seems that in the course
> of things it receives multiple QEvent::Show events. I've tried various
> way of having it run only once, but seem to be missing something.

You mentioned that you found a solution but traditionally:

// please forgive syntax, as I have not written Qt/C++ for a year or so,
// so am probably a bit rusty...
class Example {
    public slots:
        void handle_show_event(void);
        bool started_thread;  // initialize to false in the constructor

void Example::handle_show_event(void) {
    if (started_thread) return;
    started_thread = true;    // do this first to avoid a race...


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